1. Field of the Invention
The present invention relates to an apparatus for measuring thermodynamic characteristics of a hydrocarbon sample, more particularly intended for providing thermodynamic measurements relating to a deposit fluid sample coming from an oil well.
2. Description of Related Art
On discovering an oil field, samples of the deposit fluid are periodically taken in order to make it possible to measure the various physical and thermodynamic characteristics of the fluid. These measurements make it possible to characterize the deposit fluid and thus to optimize the production of the well by providing surface installations which are best suited to treating the fluids produced, by predicting the behavior of the fluid inside the deposit during production, as well as its behavior under the surface conditions as it is exploited.
The measurements taken from the sample make it possible to know the composition of the deposit fluid as well as its physical characteristics such as its viscosity cosity or its compressibility. The measurements taken also make it possible to predict the change in the composition of the gas in the deposit and to decide on the drainage and recovery mechanisms. In addition, by determining the change in the deposit fluid under the temperature and pressure conditions existing at the well bottom, the measurements make it possible to predict the production lifetime of the well. In fact, the pressure of the fluid in the reservoir rock decreases as a function of the production of the well and can thus fall to a level where the gases present in the reservoir rock start to condense.
The sampling of the deposit fluid, as well as the measurement of the characteristics of this fluid, should be carried out under very precise conditions, because the volume sampled for analysing it represents only a minute part of the fluid in the reservoir rock, and the physical properties measured can only be extrapolated validly to the deposit itself if this fraction is itself representative of the whole and if the measurements taken are exact.
There are two ways of taking samples of the deposit fluid; bottom sampling, which is generally carried out for oil deposits, and surface sampling which comprises sampling carried out at the test gas/oil separator. In the latter case, two samples are taken, one of gas and the other of oil, both under the pressure and temperature conditions of the separator, namely of the order of 40 bar and 30.degree. C., that is to say markedly less than those existing at the well bottom. The deposit fluid is then reconstituted by recombining the liquid and gas fractions under the pressure and temperature conditions of the well bottom in order to measure the various characteristics of the sample.
These measurements are normally carried out in the laboratory, remote from the drilling site, using a measurement apparatus fitted with cells in which the temperatures and pressures of the well bottom are applied. In known measurement apparatuses, the measurement cells are connected to a mercury source allowing the fluid to be pressurized. The use of mercury has obvious drawbacks, in view of its toxicity and, furthermore, mercury-based systems do not make it possible to measure the deposit fluid volumes with sufficient precision.